Our invention contemplates a frequency contouring amplifier circuit which may be applied to any situation in which it is desired to amplify certain frequencies differently from other frequencies. The circuit has particular utility in the signal processing art such as associated with music reproduction. In this specification the circuit is described with respect to its use as an equalizing amplifier for shaping the signal from a phonograph cartridge signal source. Used in the preamplifier circuit the present invention has been found to reproduce music with a higher degree of quality than has heretofore been observed. It should be remembered, however, that the circuit has wide utility beyond the high fidelity music reproduction field and is not intended to be limited to this use.
To ease certain difficulties in the recording process both with records and tapes it is known in the industry to record sound in such a way that high frequencies receive a great deal more volume or amplitude than the lower frequencies. In fact, there exists an RIAA established curve for determining the ratios between the amplitudes of the various frequency signals which has been accepted as an industry standard and is used by all manufacturers. In this curve frequencies above 2122 hertz are recorded at progressively higher amplitudes in proportion to the frequency. A second break in the curve is established from 50 hertz to 500 hertz. Thus, in effect, the music is recorded in the form of an encoded signal which must be decoded by the playing equipment before it can be properly reproduced. By far the most common way to decode the signal in the playing unit is to utilize a preamplifier circuit in which the amplified signal is fed back to the input of the amplifier in a selective manner by means of a combination of resistors and capacitors and often more complicated circuits so as to selectively supress the higher frequencies. This is known as a frequency dependent feedback system and produces only an adequate signal. Whenever any capacitive feedback system is employed it is impossible to avoid induced time lag distortion which prevents the achievement of the highest fidelity signal.
An alternative approach to eliminating some of the higher frequency signals involves a passive filter network which attenuates some of the higher energy components by selectively attenuating them with suitably chosen resistors and capacitors. Since the capacitor is not a part of the amplifying circuit there is no harmful signal distorting feedback. However, this system suffers disadvantages in that the passive network draws off a considerable amount of energy and therefore a large amount of amplification is required. It turns out that the amount of amplification required usually results in clipping the signal on the higher frequencies in order to obtain sufficient amplification for the lower frequencies. This comes about as a direct result of the RIAA accepted recording standard which results in a signal having approximately one hundred times the amplitude for the higher frequencies (near 20 KHz) than for the lower frequencies (near 20 hertz). Accordingly, in the prior art it has become common to revert to the active feedback type system and then provide complex circuits designed to eliminate the distortion effect produced thereby. Sometimes these circuits are so complex they involve hundreds of components. Since all circuit components inherently have some minimum noise and some minimum inherent nonlinearities themselves, the law of diminishing returns sets in wherein the more one trys to eliminate the distortion effects the more noise is created and the minimum distortion based on real-life nonlinearities is greatly increased. Our invention avoids these problems by the novel use of an amplification circuit in which several passive networks may be used in a seggregated manner as described hereinafter.